In 3GPP LTE (3rd Generation Partnership Project Long-term Evolution, hereinafter, simply referred to as “LTE”) and LTE-Advanced (hereinafter, simply referred to as “LTE-A”), Sounding Reference signal (SRS) is used as a reference signal for measuring uplink receiving quality (refer to Non-Patent Literature 1). To be more specific, SRS includes P-SRS (Periodic SRS) and DA-SRS (Dynamic Aperiodic SRS). For both types of SRS, SRS transmission timing is controlled according to trigger information transmitted from a base station to a terminal. However, while the P-SRS is controlled by a high-order layer, the DA-SRS is controlled by a control channel (that is, PDCCH) of a physical layer.
In order to transmit the SRS from a terminal to a base station, SRS resources (hereinafter, referred to as “common resources”) which are common to all terminals are set. A notification of these common resources is performed with the cell units. For example, if a notification indicating that the common resources are first, third and eighth subframes is performed using control information, all terminals in a cell stop transmission of data signals during a predetermined time period (specifically, a final symbol) of each of the first, third and eighth subframes, and use the time period as a transmission resource of a reference signal.
In addition, information regarding a resource (that is, parameters used to identify a resource) which is practically allocated to each terminal in the common resources includes leading subframe, set band, transmission bandwidth, frame interval at which an SRS is mapped, and transmission time or the like. Each terminal is notified of this information by a higher-order layer than a physical layer.
Furthermore, SRSs are scrambled by an orthogonal sequence in each terminal and then transmitted. Furthermore, for a terminal that performs MIMO communication introduced in LTE-A, an SRS transmitted from each antenna port is scrambled by an orthogonal sequence and transmitted. That is, SRSs transmitted from a plurality of terminals or a terminal that performs MIMO communication are code-division multiplexed and transmitted.
Here, as the orthogonal sequence, a cyclic shift sequence (CS sequence) is used. More specifically, the terminal generates a transmission sequence used by the terminal itself by applying a cyclic shift corresponding to one of cyclic shift amounts 0 to 7 notified of from the base station (that is, notified by 3 bits) to a basic sequence generated by a ZC (Zadoff Chu) sequence. To be more specific, the terminal applies a cyclic shift to the basic sequence by a cyclic shift amount×symbol length/16 (ms) notified of from the base station. FIG. 1 shows a situation in which a basic sequence is cyclically shifted by ¼ symbol. On an LTE or LTE-A uplink, an SRS is arranged for every two subcarriers. Furthermore, on an LTE or LTE-A uplink, the same waveform is repeated twice within one symbol. For this reason, a waveform obtained by a cyclic shift of (8 to 15)×symbol length/16 (ms) is identical to a waveform obtained by a cyclic shift of (0 to 7)×symbol length/16 (ms).
When SRSs are transmitted from a plurality of antennas of one terminal (that is, in a case of MIMO communication), if the base station notifies the terminal of cyclic shift amounts at all antennas, the signaling amount becomes enormous. Such a problem is solved by a method of notifying of a cyclic shift amount disclosed in, for example, Non-Patent Literature 2. According to this method, a base station and a terminal share an offset pattern about offset values of cyclic shift amounts of a second antenna port, a third antenna port, and a fourth antenna port from the cyclic shift amount corresponding to a first antenna port (hereinafter, simply referred to as “offset pattern”). Here, the offset pattern is fixed.
In this shared condition, the base station notifies the terminal of the cyclic shift amount (CS0) of the first antenna port using 3 bits. In this way, the terminal can calculate the respective cyclic shift amounts corresponding to the second antenna port, the third antenna port, and the fourth antenna port from the notified cyclic shift amount (CS0) of the first antenna port. That is, the cyclic shift amount of an i-th antenna port can be calculated from CSi=CS0+k mod 8. Here, i is an antenna port identification number (0 to 3) and k is an offset value of an antenna port with identification number i with respect to the cyclic shift amount of the antenna port with identification number 0.
FIG. 2 shows an example of correspondence table in which with regard to eight cyclic shift amount candidates of an antenna port with identification number 0, four antenna port identification numbers are associated with cyclic shift amounts corresponding to the respective antenna port identification numbers.
As is clear from FIG. 2, in the case of 4 antenna ports (that is, the case of 4-antenna MIMO transmission), the offset pattern is “0, 4, 2, 6” (for i=0, 1, 2, 3). On the other hand, in the case of 2 antenna ports (that is, the case of 2-antenna MIMO transmission), the offset pattern is “0, 4” (for i=0, 1). Here, in FIG. 2, antenna port 10 means a first antenna port when one antenna port is used. Furthermore, antenna ports 20 and 21 mean first and second antenna ports, respectively, when two antenna ports are used. Furthermore, antenna ports 40, 41, 42 and 43 mean first, second, third and fourth antenna ports, respectively, when four antenna ports are used. Using such offset patterns causes a CS interval to become a maximum between antenna ports, and when SRSs are transmitted from two antenna ports or when SRSs are transmitted from four antenna ports, the SRS demultiplexing accuracy becomes the highest. Furthermore, by causing the first two elements of the offset pattern in the case of 4 antenna ports to match the offset pattern in the case of 2 antenna ports, it is possible to use a common correspondence table for the cases of 4 antenna ports and 2 antenna ports. A common correspondence table may be used for the cases of 1 antenna port, 2 antenna ports and 4 antenna ports as well.